Having lived in SW London for the whole of my life, believe you me areas change so much! Areas that were rough are now sough after and now those sough after areas are now considered to be a bit run down.
All the schools in Streatham that were "lovely community schools", but not that academic back in the day are now pretty much all Outstanding. You have the new leisure centre and Ice Rink replacing the ones that had been there for years. Loads of restaurants with a huge variety of foods from around the world.
There have always been people smoking dope, but it's not like that doesn't happen in all other areas - I went to "The Greycoat Hospital" back in the day and staff smoked dope there in the staff room - only went to a school in Westminster as the schools in Lambeth and Wandsworth were not seen as great then and you couldn't get into a Croydon school due to the high demand!
The main problem would be Southern Rail, the trains have improved a lot with the new timetable that are the via Norbury ones, but I've heard a lot of complaints for anyone who depends on Streatham Station rather than Streatham Common or Norbury - haven't seemed to have heard any major criticism or complements for trains via Streatham Hill. Streatham Common station is accessible with lifts, where as Streatham Station isn't if that's something to be considered. There are loads of buses that are heavily used.
There has been a consultation to possibly get in residential parking in Streatham so if you are a driver and want to be on the safe side, look for a rare house with a drive as if it does happen, it'll be the most expensive residential parking in London based on what I've read.